This invention relates to a signal device for turbo engines. For various special measurements on turbo-engines such as for measuring the blade vibrations, highly precise timed trigger pulses or position signals are required. The trigger pulses or position signals originate from the moving blades of the turbo-engines. The moving blades are operated at comparatively high rotational speeds. Optical probes are known to be used for this purpose. The blades travel through an illuminating beam which is correspondingly optically processed by the probe, in which case the trigger pulses are obtained from the light which is reflected by the blades on defined surfaces, for example, the blade face. The light is reflected partially in a diffuse and partially in a targeted manner. The signal rise or drop, which can be displayed by an optoelectronic receiver, is the result of the time which a defined section of the blade requires for passing through the illuminating beam.
Using a known conventionally constructed optical probe, which includes an imaging lens system, only a relatively small, virtually punctiform, illumination spot is generated on the defined blade surface in order to obtain steep signal flanks. Although characteristic signals with relatively steep flanks are already obtained from the received reflected light, it is a significant defect of the conventional probe that the imaged signals are very structured and, from one rotation of the rotor to the next, are characterized by a clearly different shape such that only trigger signals which have a poor time-related assignment to the blade position can be derived. Optical imaging using lenses in a receiving branch of the probe results in a comparatively small effective solid angle within the receiving cone of the reflected light. Furthermore, a considerable portion of the effective solid angle is reduced even further by being covered by lenses of the transmitting branch.
The punctiform, extremely small illumination spot leads to very changeable, non-reproducible reflection directions of the proportion of light which is reflected in a targeted manner. This is true particularly since, even by slight displacements of the illumination spot on the blade, different surface structures are passed through the illumination spot during each rotation. In order to achieve the required small illumination spot diameters, a coherent light source is used. However, as a result of interference, an intensity pattern (speckle pattern) is created in the reflected light which also depends on the surface structure of the blade and therefore varies with respect to place and time. For the above-mentioned reasons, for example, operationally caused radial and/or axial displacements of the rotor relative to the housing and, therefore, relative to the lighting beam result in pronounced changes of the signal shape.
German Patent document DE-OS 37 00 777 relates to an arrangement by which the rotating or moving condition of an object is measured in a highly precise manner. The arrangement consists of a reference position generator (l/rotation generator) and of a so-called "encoder" by which the position of the object is measured relative to the reference positions. The encoder operates using a diffraction grid which is firmly mounted on the object. The reference position generator operates according to the following principle. A rectangular reflection label mounted on the object is irradiated by a convergent luminous beam whose dimension in the area of the label is approximately twice as large as the label. Because of the convergence of the luminous beam, the reflected light, while the label passes through the luminous beam, travels through a defined angular area. Two correspondingly arranged photocells therefore see the reflected light in a time-staggered manner, and that label position is defined as the reference position at which both photocells indicate the same intensity.
In this known case, a cylinder lens is used, among others, in order to image an elongated scanning spot which is similar to the label on the reflection or reference label, in which case a longitudinal course of the reflection label is present which extends perpendicularly to the moving direction.
This operating principle requires that the reflection label be planar and reflect in a specular manner (the angle of incidence being equal to the angle of reflection). This requirement cannot be met, particularly for blade faces which have ground structures. In this case, the light would be scattered in a more or less diffuse manner and, as a result of the surface structure (scratches, striae), in directions that vary considerably from place to place. Polishing the faces would result in an additional cost-intensive operation. Also, it would have to be expected that, after the blades graze the inlet coatings of the housing during their operation (which frequently occurs, for example, in the case of compressor pumps), considerable grinding traces would again be present on the blade faces.
An arrangement known from German Patent document DE-AS 14 63 050 relates to photoelectric cutting line scanning analogous to the reflection and reference label scanning according to German Patent document DE-OS 37 00 77. In this case, the illuminating beam is expanded in one dimension in parallel to the cutting lines (fracture edges) of a glass band which are detected in order to stress the cutting lines on the moving glass band with respect to existing structures, such as dust or streaks in the glass. A cylinder lens is arranged in front of a linear luminous surface as the light source. A photocell is assigned to a fiber optical waveguide plate (fiber optical waveguide).
U.S. Pat. No. 5,201,227 relates to an arrangement for measuring the blade vibrations of rotating moving blades of a gas turbine engine. For this purpose, a probe is fixed to the turbine housing in such a manner that the radially interior probe end (face side) is arranged radially at a distance above the moving blade tips. The moving blade scanning takes place using an illuminating fiber optical waveguide which is arranged in the probe in an axially central manner and by using a receiving fiber optical waveguide which is arranged concentrically with respect to it. On the corresponding end of the probe head, the illuminating fiber optical waveguide includes a small diameter lens which is followed axially at a distance in the transmitting and receiving branch by a large diameter lens. A cover disk, which is situated on the probe face, has only a purely protective function. The arrangement results in a comparatively small receiving solid angle covered by the probe above the corresponding moving blade. The probe will therefore only be able to receive small proportions of the light reflected by the blade faces, in which case, in addition--as a cause of the given surface structures of the moving blade--only light reflections which "go randomly back and forth" and which furnish comparatively impure signals would be received.
Because of the construction of the above-described probe, the illumination spot on the blade is circular. This results in the following disadvantages. If the spot diameter were very small, because of the blade surface structure, very structured signals would be obtained which change from one rotation to the next and have the corresponding disadvantages with respect to the imprecision of the triggering. If the spot diameter were larger, an averaging could take place by way of the structures but, at the same time, the signal flanks would be flatter. This also has the disadvantages with respect to the triggering precision.
There is therefore needed a signal device for turbo-engines by which, particularly as the result of the construction and arrangement of an optical probe, while the turbo-engine is operating, signals can be generated by way of the moving blades. The signals should be optimally reproduced from one rotation of the rotor to the next and be largely uniform with a view to operational influences and definitions.
These needs are met according to the present invention by a signal device for turbo-engines which generates signals when moving blades on a rotor pass through a defined circumferential position of the housing. The signals are precise with respect to time. The signal device includes a probe which has fiber optical waveguides for emitting and receiving light and which emits an illuminating beam focussed in the direction of the rotor. The probe is arranged on the housing at a distance with respect to the free faces of the moving blades. The probe emits an illuminating beam with an elliptical cross-section, through which the moving blades travel, in such a manner that, on each corresponding blade face, an elliptical illumination spot is imaged whose large axis is aligned essentially in parallel to the delivery-side or suction-side edges of the faces. The signals are generated from the light reflected by the blade faces.
Through the use of the device according to the present invention, signals can be made available which, among others, have the following advantageous characteristics:
1) The signals are slightly structured; i.e., the signal shape is largely unimpaired by the surface structure of the moving blade faces given on the material side and possibly by the mechanical machining (grinding) or wearing-out (on the inlet coatings);
2) The signals can be reproduced from one rotation to the next;
3) The signals have steep signals flanks; and
4) The signals have slight intensity fluctuations in the case of a distance between the probe and the blade which can be varied in a relatively large defined area.
The device according to the present invention is preferably suitable for use on axial-flow compressors or turbines of turbo-engines, particularly gas turbine engines. In a radial bore of the compressor housing or turbine housing, the optical probe is fixed at the desired distance of preferably 0.5 to 2.5 mm from the blade faces. According to the invention, during each blade pass through the illuminating beam, an elliptical illumination spot is imaged on the respective blade face. Before it is fastened on the housing, the probe must only be rotated in the circumferential direction to such an extent that the large axis of the elliptical illuminating beam is aligned locally in parallel, for example, with respect to the delivery-side edge, on a blade face, and the small elliptical axis (smallest beam diameter) therefore extends transversely to the respective edge. In this case, it is a prerequisite that, during the construction of the probe, the illuminating beam be aligned with the course of the probe axis by way of the illuminating--particularly, monomode--fiber optical waveguide contained in the probe. The signal rise or the signal fall is therefore the result of the time required for the delivery-side or suction-side edge of the concerned blade face to pass through the elliptical illuminating beam.
The expansion of the illuminating beam in parallel to the respective blade edge leads to a considerable reduction of the influence of the surface structure of the blade on the received signal obtained from the reflected light. The endeavored rise rate of the signal in this case is not influenced in a disadvantageous manner. The indicated focussing of the illuminating beam transversely with respect to the respective edge results in short rise times of the probe signal.
The construction and the arrangement of the cover disk in the receiving part or branch of the probe allows for a large receiving solid angle. This is because light reflected by the blades can be received by a relatively large disk surface. As a result of the indicated edge-parallel expansion of the illuminating beam, an advantageous construction is possible (adjusting of the fiber optical waveguide for the probe illumination). In addition, an advantageous optical separation of the transmitting and the receiving portion or branch of the probe using the arrangement and construction of a tube-shaped screen within the recess which, in turn--despite the screen--can be uniformly illuminated for the most part by the received light.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.